In the vehicular industry, various environmentally-friendly vehicles have been developed with the object of reducing a discharge amount of carbon dioxide to 95 g/km, which is 27% of the current amount thereof by 2021 based on European regulations. Furthermore, vehicle makers strive to develop technology to downsize and improve fuel economy in order to satisfy 54.5 mpg (23.2 km/1), which is a regulation for corporate average fuel economy (CAFE) in the USA by 2025.
In particular, high-performance and high-efficiency technology for engines and transmissions for maximizing fuel economy of vehicles has been developed, and this technology includes an increase in the number of gears, novel concept driveaway devices, high efficiency two-pump systems, fusion hybrid technology, technologies relating to an automatic/manual fusion transmission and a hybrid transmission, and the like.
Specialized steel for uses in transmissions is used in a carrier, a gear, an annulus gear, shafts, a synchronizer hub, and the like of the transmission. A use ratio of the specialized steel is currently about 58 to 62 wt % based on the total weight of the steel. For example, in a pinion shaft, a needle bearing, and an engine valve train-based roller swing arm of the transmission and the like, there is a continuous demand for developing a high-strength and high-durability material due to the requirements of reducing weight and downsizing, and until now, an SUJ2 steel containing 1.5 wt % of chromium (Cr) has been used.
However, because of increased severity of conditions due to downsizing and also a size reduction of parts such as the bearing and the like, durability of the material is reduced which causes damage to the surface and, when there is no lubrication, increases a surface temperature and reduces hardness in a high-temperature and high-revolution environment.
For example, for a bearing serving to fix a rotation shaft to a predetermined position, support a weight of the shaft and a load applied to the shaft, and rotate the shaft, a repeated load is applied in proportion to a rotation number. In order to endure the repeated load, fatigue resistance, wear resistance, and the like are required.
Generally, a bearing steel is subjected to steelmaking in a converter or an electric furnace, refined in a ladle while a strong reducing atmosphere is maintained to reduce an amount of a non-metallic inclusions, refined in a state where an oxygen content is reduced to 12 ppm or less through a vacuum degassing process, thereafter, solidified into a cast slab or a steel ingot by a casting process, subjected to crack diffusion treatment in order to remove segregation and large carbides existing at the center of the material, and rolled.
Thereafter, an intensely slow cooling operation is performed in order to soften the material in a rolling factory to produce a bearing steel wire rod or rod material, and the produced wire rod is produced into bearing products through spheroidizing heat-treating, forging, quenching, tempering, and grinding processes and the like.
During the aforementioned manufacturing process, a spheroidizing heat-treating process is mainly performed by diffusion at high temperatures, and globular particles are grown through a process similar to an Ostwald ripening principle to form a spheroidized tissue.
However, because the spheroidizing process is a process requiring growth of the globular particles, a long time is spent for spheroidizing, and thus manufacturing cost is increased. It is difficult to secure sufficient strength and durability life due to an increase in severity of the bearing because of downsizing, size reduction, and the like.